1. Field of the Invention
The present invention relates to a super twisted nematic mode liquid crystal display (hereinafter, abbreviated as an "STN-LCD") with color compensation which is used for, for example, a monochrome display or a multicolor display in a word processor, or a personal computer.
2. Description of the Related Art
In such an STN-LCD, a display is performed by utilizing the birefringence of super twisted nematic (STN) liquid crystal. Due to the anisotropy of refractive index, the display is colored so as to be yellow, green or blue, which is referred to as a coloring phenomenon. In order to compensate for the coloring phenomenon, the STN-LCD is generally provided with a color compensating plate.
A double-layered STN-LCD (DSTN-LCD) is a representative example in which such a color compensating plate is provided. In the DSTN-LCD, the coloring which occurs in displaying cells constituting a first layer is color-compensated by using optical compensating cells constituting a second layer. The optical compensating cells of the second layer have a twisted structure which is reversed to a twisted structure of the cells of the first layer, whereby an achromatic color display is realized.
Alternatively, there is a proposal to use a phase plate for the same optical compensation as in the case where the above optical compensating liquid crystal cells are used. An exemplary STN-LCD with a phase plate is disclosed in Japanese Laid-Open Patent Publication No. 64-519. In the disclosed STN-LCD, the phase plate is produced by uniaxially extending a polymeric film of polyester, polyvinyl alcohol, or the like, and a display is made to be achromatic by the thus produced phase plate.
FIGS. 17A and 17B show the STN-LCD disclosed in the above-identified patent publication. FIG. 17A shows an STN-LCD of a twentieth example, and FIG. 17B shows an STN-LCD of a twenty-first example of the above-identified patent publication. The STN-LCD shown in FIG. 17A includes a front-side polarizing plate 1, a front-side phase plate 2 of a polymeric film which is uniaxially extended, a pair of light transmission substrates 4 for an STN liquid crystal panel 10, a pair of transparent electrodes 5, a pair of orientation films 6, a nematic liquid crystal layer (an STN liquid crystal layer) 7 to which a chiral dopant is added, and a back-side polarizing plate 8. The STN-LCD shown in FIG. 17B additionally includes a back-side phase plate 9 of a polymeric film which is uniaxially extended. In the STN-LCDs shown in these figures, light is incident on the STN liquid crystal panel 10 from the back-side polarizing plate 8. As described above, in the STN-LCD of the twentieth example, one phase plate of a uniaxial polymer film is provided in front of the STN liquid crystal panel. In the STN-LCD of the twenty-first example, two phase plates are provided in front of and at the back of the STN liquid crystal panel, respectively.
Moreover, a thirty-first example to the thirty-fourth example of the above-identified patent publication describe, in order to perform the optical compensation, a construction in which one liquid crystal polymeric film of a mixture film of polypeptide and polymethylmethacrylate which exhibits a cholesteric phase and has an inversely twisted structure by 210.degree. to 360.degree., instead of the phase plate is disposed.
However, the conventional DSTN-LCD necessitates two layers of cells for displaying and compensating. Thus, the DSTN-LCD has a problem in that the thickness thereof is larger and also the weight is larger than that of a single-layer STN-LCD.
In addition, the construction as shown in FIG. 17A or 17B in which one phase plate of a uniaxially extended polymeric film is provided in front of the STN liquid crystal panel, or two phase plates of uniaxially extended polymeric films are provided in front of and at the back, of the STN liquid crystal panel has a limitation in an achromatic degree of the display. Thus, there exists a problem in that such a construction is inferior to the DSTN-LCD in the contrast ratio, and the black/white display. This is because the phase difference variation depending on the wavelength in the liquid crystal panel is not completely equal to the phase difference variation depending on the wavelength in the uniaxially extended polymeric film, so that the phase difference cannot be compensated over the entire visible range. Another reason is that the rotatory polarization of the liquid crystal panel cannot be compensated unless a plurality of phase plates are disposed one over the other in such a manner that the optical axes are rotated from each other so as to make a spurious twisted structure.
Moreover, according to the construction in which one liquid crystal polymeric film which exhibits a cholesteric phase and has an inversely twisted structure is disposed in front of the STN liquid crystal panel, instead of the phase plate, a good display to a degree closer to that attained by the DSTN-LCD can be obtained. However, it is difficult to equalize the phase difference variation depending on the wavelength in the STN panel with the phase difference variation depending on the wavelengths in the liquid crystal polymeric film, so that the construction also has a limitation in achromatizing the display. In addition, the construction has various problems in a method for strictly regulating the twist angle by 180.degree. or more, in reliability for environmental resistance, and in high-volume production capability, so that this construction is not practically used.